1. Field of the Invention
This invention relates generally to a controllable by-pass circuit for parallel connected battery modules or cells and, more particularly, to a control circuit that is able to by-pass connected battery cells or modules that are part of a larger battery system and control the discharge and charge current.
2. Discussion of the Related Art
Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as the extended range electric vehicles (EREV) that combine a battery and a main power source, such as an internal combustion engine, fuel cell systems, etc., and electric only vehicles, such as the battery electric vehicles (BEV). All of these types of electric vehicles employ a high voltage battery that includes a number of battery cells. These batteries can be different battery types, such as lithium ion, nickel metal hydride, lead acid, etc. A typical high voltage battery system for an electric vehicle may include a large number of battery cells or modules to meet the vehicle power and energy requirements. The battery system can include individual battery modules where each battery module may include a certain number of battery cells, such as twelve cells. The individual battery cells may be electrically coupled in series, or a series of cells may be electrically coupled in parallel, where a number of cells in the module are connected in series and each module is electrically coupled to the other modules in parallel. Different vehicle designs include different battery designs that employ various trade-offs and advantages for a particular application.
A battery cell in a battery may fail or may otherwise be limited in performance for a number of reasons, such as an internal short, loss of capacity, high resistance, high temperature, etc. A vehicle battery pack typically includes a variety of sensors and other diagnostic devices that can determine if the battery performance is limited, is failing or may fail in the near future. Because the battery cells may be electrically coupled in series, failure of one cell in the series may prevent use of other cells in the series and may result in vehicle shut-down. Therefore, the battery can be disconnected from the circuit before a major battery failure has occurred and warning lights can be provided indicating such a failure.
Depending on the battery type, such a failure may result in a walk-home condition where the vehicle needs to be towed and cannot be driven. It would be desirable to provide a circuit where individual cells of a battery can be switched out of the circuit so that the walk-home condition and other scenarios may be avoided.